Method of preparing equipment for holding molten metal

ABSTRACT

Method of preparing equipment for holding molten high melting, chemically active metal such as titanium, molybdenum or the like in a crucible or mold, including the prevention of formation of impurities and non-metal inclusion in the molten metal, by providing a receptacle of refractory material as cast, extruded or welded part exhibiting microscopic pores, but providing only insignificant leakage at room temperature; at least a portion of the outer wall surface of the receptacle is lined with a protective layer of copper, chromium, nickel or tin for sealing the pores and for covering at least part of the wall surface of the receptacle with that lining to establish a dense integral pore-free layer that is integral with the sealing of the pores, the lining may also provide corrosion proofing if the receptacle is immersed in a corroding fluid (e.g., potossium or sodium or the like). The receptacle is then filled with the metal and sealingly closed at least when the metal is in the liquid state.

154] METHOD OF PREPARING EQUIPMENT FOR HOLDING MOLTEN METAL inventors: Hans Joachim Wallbaum; Wolfgang Hornschemeyer, both of Osnabruck,

Germany Kabel-und Metallwerke Gutehoffnungshutte Aktiengesellschatt Filed: Sept. 4, 1970 Appl. No.: 69,824

Assignee:

[30] Foreign Application Priority Data Sept. 15, 1969 Germany ..P 19 46 598.6

U.S. Cl. ..75/10 R, 204/25 Int. Cl. ..C2'-2d 7/00, C23b 5/56 Field of Search ..204/16, 23, 24, 25, 26;

75/10 C, 10 V, 10 R References Cited UNITED STATES PATENTS FOREIGN PATENTS 0R APPLICATIONS idiots 9/1960 Australia ..204/26 39/14354 7/1964 Japan ..204/25 Primary Examiner-.lohn l-l. Mack Assistant Examiner'T. Tufariello Attorney-Smyth, Rost'on & Pavitt and Ralf H. Siegemund [57] ABSTRACT Method of preparing equipment for holding molten high melting, chemically active metal such as titanium, molybdenum or the like in a crucible or mold, including the prevention of formation of impurities and nonmetal inclusion in the molten metal, by providing a receptacle of refractory material as cast, extruded or welded part exhibiting microscopic pores, but providing only insignificant leakage at room temperature; at least a portion of the outer wall surface of the receptacle is lined with a protective layer of copper, chromium, nickel or tin for sealing the pores and for covering at least part of the wall surfaceof the receptacle with that lining to establish a dense integral pore-free layer that is integral with the sealing of the pores, the lining may also provide corrosion proofing if the receptacle is immersed in a corroding fluid (e.g., potossium or sodium or the like). The receptacle is then filled with the metal and sealingly closed at least when the metal is in the liquid state.

8 Claims, 1 Drawing [figure 3,713,808 Jan. 30, 1973 METHOD OF PREPARING EQUIPMENT FOR HOLDING MOLTEN METAL The present invention relates to a method for preparing equipment for holding molten metal, such as a crucible or mold. Metals such as molybdenum, titanium or zinconium are characterized by both, a high degree of reactivity and high melting temperatures. These metals are melted, for example, by means of a 1 vacuum furnace, using a. crucible consisting of copper or copper alloys. In order to avoid influx of gas, the

crucible is sealed vacuum tight. Highly alloyed steels are also smelted in that manner, particularly to avoid liquation and to obtain a high degree of purity so as to improve the characteristics of the products made from the melt.

Problems arise when the crucibles are castings, as the walls are likely to exhibit microscopic pores which are still tight at room temperature, but at operating (smelting) temperatures, the crucible does admit gas such as air, and the vacuum deteriorates accordingly. As a consequence, the molten metal will have nonmetallic inclusions such as oxides and that, in turn, may render the metal unusable;

Crucibles made of sheet metal, e.g., copper, exhibit similar leakage at high operating temperatures, because these crucibles usually have welded joints, and they were found occasionally to admit gas.

The problem solved by the present invention is to find a leak proof crucible. Thus, it is an object of the invention to provide a hollow implement, such as a mold or crucible that is and remains leak proof, and is made of a material of high thermal conductivity. In accordance with the preferred embodiment of the invention, it is suggested to provide the receptacle of refractory material in accordance with the usual techniques, exhibiting, as a consequence, microscopic pores at least in parts of its wall structure. An outer lining is provided next to serve as leak proofing and/or corrosion proofing jacket. Particularly, the lining seals the pores and covers the sealed pores and adjacent surface parts with an integral, fissure free lining. Thus, a crucible may be constructed as usual, for example, as a cast container (tube etc.), and the outer surface is clad subsequently with a lining that leak-proofs the crucible. In the preferred form, the lining is provided by means of electroplating, for example, copper and the resulting layer should have not more than 1 to 2 mm thickness. Electroplating was found particularly sutiable to buildup an outer lining that seals the pores and is developed as an integral layer having no porosity whatever.

It was found that crucibles clad in that manner do not exhibit leakage any longer, particularly not at the high operating temperatures needed for melting titanium etc. The maximum tolerable leakage rate is, e.g., '10 torr/sec. It was found in particular, that the leakage rate in the crucible improved in accordance with the invention, remained well below that maximum rate. For example, the leakage rate was reduced by two orders of magnitude, down to 4'10 torr/sec. and less.

A copper layer exhibits the additional advantage of corrosion proofing the vessel. This is particularly important in case a crucible is cooled from the outside, not by means of water but by a molten mixture of sodium and potassium. In case of cooling the outer layer may well consist of a lower melting material which extends the range of available material, e.g., a tin layer may be deposited on the outer surface of the crucible.

It should be noted that in the past some crucibles have been made by means of galvanic techniques. However, crucibles of the type alluded to above require .wall thickness which cannot be economically produced by means of electroplating particularly as that would require too long a time. However, galvanic production of the particular cover is quite feasible.

While the specification concludes with claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention, it is believed that the invention, the objects and features of the invention and further objects, features and advantages thereof will be better understood from the following description taken in connection with the accompanying drawings in which:

The FIGURE illustrates a section view through a crucible prepared in accordance with the invention for melting high melting metal, such as titanium.

Turning now to particulars, the FIGURE shows a crucible 3 disposed in a vessel 2 that contains a cooling medium 1, such as a mixture of sodium and potassium. This coolant is highly reactive. On the other hand, the crucible is designed to melt, for example, titanium under vacuum or other high melting metals. Thus, the crucible must be both, corrosion proof and leak proof. The crucible itself is made as a cylindrical tube which may have been cast, extruded or metal sheet has been appropriately shaped and welded. As a consequence of preparing the crucible in this manner, its cylindrical wall has, at least in parts, microscopic pores.

The tubular wall of the crucible has 'a bottom flange 4 to which is secured a cover 5 serving also an electrode for melting, by means of electrical energy. The cover may be sufficiently thick and may have been made in a manner and of a material that does not produce pores. The upper, open end of the tube has a flange 6'provided with a cover plate 7, closing the crucible after filling. An electrode 9 traverses the plate but is vacuum tight sealed. Evacuation studs may be provided separately as is conventional. Titanium 8 has been placed in the crucible to be molten by means of an arc as it will develop between electrode 9 and the metal 8, due to conductive connection thereof to bottom cover 5.

In order to avoid entry of gas through microscopic pores and fissures in the walls of the crucible, particularly at elevated temperatures, the crucible is covered with a layer 10 to leak-proof and to corrosion-proof the crucible. In case crucible 3 was made from sheet metal, copper wire with particular tin content may have been used for welding. This establishes an additional reason for corrosion proofing the crucible, as sodium and potassium corrodes strongly any material that contains t1n.

Layer 10 has preferably been disposed on the cylindrical outer wall of tubular crucible 3 by means of electroplating of copper. This plating process sealed the pores and covered the sealed pores with an integral copper layer, having dense, nonporous, fissure free texture. The copper layer is one or a few millimeter thick, the walls of the crucible, of course, are considerably thicker.

The invention is, of course, not limited to this particular type of melting, nor are principles of the invention limited to crucibles. Induction furnaces, for example, for melting or alloying, or molds for casting of metal under vacuum, can be made and improved in the same manner.

In other words, the principle of the invention resides in the preparation of a receptacle for vacuum melting or the like, such as a mold, crucible or the like, so that its interior wall structure is particularly adapted to the intended purpose (high refractory material etc.) That receptacle is then provided with an outer layer. For most instances, the principle function of the layer is to seal microscopic leaks'in the vessel, particularly of the type that will become effective at elevated operating temperatures. In addition, it may be required to corrosion proof the outer surface of the vessel particularly because the surrounding agent may tend to corrode the vessel, and that tends to increase leakage. Therefor, corrosion proofing, such as in the illustrated example, is (or may be) an indirect measure to leak proof the vessel, particularly on a long term basis.

The invention is not limited to the embodiments described above but all changes and modifications thereof not constituting departures from the spirit and scope of the invention are intended to be included.

We claim: 1. Method of preparing and using equipment for holding molten metal, such as providing a crucible or mold, and including preventing formation of impurities and non-metal inclusion in the molten metal, comprisproviding a receptacle such as a crucible or mold of refractory material as cast, extruded or welded part exhibiting microscopic pores, exhibiting only insignificant leakage at room temperature, the receptacle to have an opening;

electroplating a protective layer on at least a portion of the outer wall surface of the receptacle, for sea]- ing the pores and for covering at least part of the wall surface of the receptacle with that lining to establish a dense integral pore-free layer that is integral with the sealing of the pores;

filling the receptacle with the metal sealingly closing the opening of the receptacle, at least when the metal is in the liquid state; and

melting the metal in the receptacle.

2. Method as in claim l,-the electroplating step comprising the step of electroplating the receptacle to obtain a lining of one or a few millimeters thick.

3. Method as in claim 1, the lining step comprising electroplating the receptacle with copper, chromium or nickel.

4. Method as in claim 1, the lining step comprising lining the outer surface of the receptacle with a layer of relatively low melting point.

5. Method as in claim 1, providing the receptacle as a hollow cast part, and plating at least part of the outer surface of the cast receptacle for sealing micro-pores in the cast part.

6. Method as in claim 1, providing the receptacle from sheet metal stock, that is being welded to obtain surface of the receptacle including the welding seams for sealing the seams and adjoining areas.

7. Method as in claim 1, the lining step comprising providing the outer surface with a layer of tin.

8. The method of preparing equipment for melting high melting metal such as titanium, molybdenum, zirconiurn or the like, comprisin the steps of:

providing a crucible of re ractory materials exhibit- 

1. Method of preparing and using equipment for holding molten metal, such as providing a crucible or mold, and including preventing formation of impurities and non-metal inclusion in the molten metal, comprising: providing a receptacle such as a crucible or mold of refractory material as cast, extruded or welded part exhibiting microscopic pores, exhibiting only insignificant leakage at room temperature, the receptacle to have an opening; electroplating a protective layer on at least a portion of the outer wall surface of the receptacle, for sealing the pores and for covering at least part of the wall surface of the receptacle with that lining to establish a dense integral pore-free layer that is integral with the sealing of the pores; filling the receptacle with the metal sealingly closing the opening of the receptacle, at least when the metal is in the liquid state; and melting the metal in the receptacle.
 2. Method as in claim 1, the electroplating step comprising the step of electroplating the receptacle to obtain a lining of one or a few millimeters thick.
 3. Method as in claim 1, the lining step comprising electroplating the receptacle with copper, chromium or nickel.
 4. Method as in claim 1, the lining step comprising lining the outer surface of the receptacle with a layer of relatively low melting point.
 5. Method as in claim 1, providing the receptacle as a hollow cast part, and plating at least part of the outer surface of the cast receptacle for sealing micro-pores in the cast part.
 6. Method as in claim 1, providing the receptacle from sheet metal stock, that is being welded to obtain hollow configuration, and plating at least part of the surface of the receptacle including the welding seams for sealing the seams and adjoining areas.
 7. Method as in claim 1, the lining step comprising providing the outer surface with a layer of tin. 